Radio repeater trunking (i.e., time sharing of a single repeater communications channel among many users) is well known. Although there are many actual potential applications for trunked radio repeater systems, one of the more important applications is for public service trunked (PST) systems. Various known prior trunked communication system of this sort offer the capability of handling high speed digital data communications (e.g., digitally encrypted voice) in addition to conventional analog voice communications and, thus, provide users with selectable digital access to the trunked system. An exemplary digital access PST system is discussed in,commonly-assigned U.S. Pat. No. 4,905,302 to Childress et al entitled "Trunked Radio Repeater System," which is incorporated by reference herein.
As is well known, it may not be possible for a single RF repeater transmitting site to satisfactorily serve a geographically large coverage area. Accordingly, systems which must provide RF communications for an entire large geographical area (e.g. , a major metropolitan area, a large county, etc.) typically include multiple RF transmissions sites. FIG. 1 is a schematic diagram of a simplified multiple-site "simulcast" system having three radio repeater (transmitting) sites S1, S2 and S3 for providing simulcast communications to geographical coverage areas A1, A2 and A3, respectively. A control point or "hub" C (e.g., a dispatch center) provides identical signals to each of sites S1 through S3 via links L1 through L3, respectively. These links are typically microwave inks. Each site S1-S3 transmits the signals it receives from the control point C to its respective coverage area A, so that a mobile or portable transceiver receives the same signal no matter where it happens to be in the communications system overall coverage area A (which constitutes the "union", in an analogy to Venn diagrams of the individual coverage areas A1, A2 and A3).
Mobile or portable transceivers within area A1 can receive the signals transmitted by site S1, transceivers within area A2 can receive the signals transmitted by site S2, and transceivers within area A3 can receive signals by site S3. Transceivers moving out of a first site coverage area and into a second sites area cease monitoring the signals transmitted by the first site and begin monitoring the signals transmitted by the second site such that communication is continuously maintained without interruption so long as the transceiver stays within the overall combined system coverage area A'. In order to insure the RF transmission from these multiple sites do not interfere with each other in overlapped regions, it is necessary to: a) operate the sites on different frequencies, or b) operate the sites on the same frequency and time share their operation, or c) employ coherent simulcast techniques. In a coherent simulcast system, the various transmitting sites transmit substantially the same signals substantially simultaneously at substantially the same radio frequency to avoid adverse interference effects at the mobile transceiver when in the overlap areas.
More specifically, simulcast involves transmitting "coherent" modulation over several transmitters operating on the same assigned RF channel but located at different sites. This means that the modulated RF signals from two simulcast sites would have to arrive at a receiver in the overlap coverage area at precisely the same time. The receiver would then see no difference between the signals received from the two simulcast sites. The demodulated output from the receiver would appear as if it were receiving a signal from only a single site. To elaborate, in a simulcast system, there are two or more identifiable geographical areas commonly referred to as the "capture" and the "non-capture" or "overlap" zones. A capture zone is defined as an area in which the carrier level of one transmitter exceeds the second by approximately 10 dB or more. In this area, the mobile receiver will receive the stronger signal to the complete or nearly complete exclusion of the weaker signal. The capture zone provides the best audio quality in the system.
In the "non-capture" or "overlap" zone, the mobile receive cannot be captured by a single transmitter. Within this zone a mobile receiver accepts two or more carrier signals. The carrier signals mix randomly producing a stronger or weaker signal. If the power level difference between the received carrier signals is less than 6 dB with voice modulation, audio intermodulation and distortion products may be created. Audio distortion increases to a maximum when the carrier signal levels are equal.
The prime concern in the "overlap" zone is to assure that the received carrier signals, at worst, do not detract from one another and at the best, reinforce one another. In addition, the transmitted audio from each carrier signal must be equalized to reduce the distortion produced at the mobile receiver. In general, to achieve these objectives three types of system equalizations are required: RF carrier frequency, transmitter modulation level and transmitter modulation phase.
Each radio channel at all sites is modulated with amplitude, phase and time delay corrected information. To accomplish this time, phase and amplitude stable circuits must be provided between a main control point site and all other simulcast transmit sites by means of a phase stable (radio, microwave or fiber optic) back-bone system. Commercial wire-common-carriers do not provide the stability required for simulcast. Dedicated, user controlled, voice/data grade, synchronous multiplex used in conjunction with radio, microwave or fiber optic back-bone distribution paths most effectively provide needed circuits and stability for simulcast.
Because path lengths to the various simulcast sites vary and audio frequency and phase response of each transmitter is different, independent time, phase, and frequency response equalization of each simulcast path must be provided. Moreover, the equalization must be relative to the path having the longest unequalized time delay (which is usually, but not necessarily, the longest physical path).
An exemplary public service trunking simulcast system of this sort, which also provides digital access capabilities, is disclosed in greater detail in commonly-assigned U.S. Pat. No. 5,172,396 issued Dec. 15, 1992 to Rose, Jr., entitled "Public Service Trunking Simulcast System", and has been successfully in public use for quite some time.
Simulcasting in a multiple-site RF transmission is thus generally known. The following list (which is by no means exhaustive) of prior issued patents describe various aspects of RF transmission simulcasting and related PST issues:
U.S. Pat. No. 5,172,396 to Rose et al. PA1 U.S. Pat. No. 4,696,052 to Breeden; PA1 U.S. Pat. No. 4,696,051 to Breeden; PA1 U.S. Pat. No. 4,570,265 to Thro; PA1 U.S. Pat. No. 4,516,269 to Kurnock; PA1 U.S. Pat. No. 4,475,246 to Batlivala et al; PA1 U.S. Pat. No. 4,317,220 to Martin; PA1 U.S. Pat. No. 4,972,410 to Cohen et al; PA1 U.S. Pat. No. 4,903,321 to Hall et al; PA1 U.S. Pat. No. 4,608,699 to Batlivala et al; PA1 U.S. Pat. No. 4,918,437 to Jasinski et al; PA1 U.S. Pat. No. 4,578,815 to Persinotti; PA1 U.S. Pat. No. 5,003,617 to Epsom et al; PA1 U.S. Pat. No. 4,939,746 to Childress; PA1 U.S. Pat. No. 4,905,302 to Childress et al; PA1 U.S. Pat. No. 4,905,234 to Childress et al; PA1 U.S. Pat. No. 4,093,262 to Dissosway et al; PA1 U.S. Pat. No. 4,926,496 to Cole et al; PA1 U.S. Pat. No. 4,968,966 to Jasinski et al; PA1 U.S. Pat. No. 3,902,161 to Kiowaski et al; PA1 U.S. Pat. No. 4,835,731 to Nazarenko et al; PA1 U.S. Pat. No. 4,218,654 to Ogawa et al; PA1 U.S. Pat. No. 4,255,814 to Osborn; PA1 U.S. Pat. No. 4,411,007 to Rodman et al; PA1 U.S. Pat. No. 4,414,661 to Karlstrom; PA1 U.S. Pat. No. 4,472,802 to Pin et al; PA1 U.S. Pat. No. 4,597,105 to Freeburg; and PA1 Japanese Patent Disclosure No. 61-107826.
U.S. Pat. No. 5,172,396, issued Dec. 15, 1992 to Rose et al., entitled "Public Service Trunking Simulcast System", discloses a trunked radio simulcast system having control site and remote site architectures that include RF transmission timing synchronization features that are relevant to the presently preferred exemplary embodiment. In addition, U.S. Pat. No. 4,903,321, issued Feb. 20, 1990 to Hall et al., entitled "Radio Trunking Fault Detection System," discloses a trunked radio repeater system having a radio frequency repeater site architecture that includes fault and call testing and failure detection features that are somewhat relevant to the present invention. These patents are both commonly assigned to the assignee of the present invention and are both incorporated by reference herein.
The present invention is directed toward a "narrow band" PST simulcast radio frequency transmission system having high-speed and low-speed digital access capabilities in addition to analog voice that operates in the 900 MHz frequency range, as distinguished from "wide band" systems that operate predominantly in the 800 MHz frequency range. In particular, the present invention is directed toward a method and apparatus for handling low speed data distribution between sites in a narrow band PST simulcast system.
Conceptually, the 900 MHz narrow band simulcast system in accordance with the presently preferred embodiment of the invention, may be envisioned as a hardware addition or overlay to the applicant's 800 MHz PST repeater systems and PST simulcast system (e.g., see the Childress '746, Childress '302 and Rose et al. '396 patents mentioned above). Moreover, an important aspect of the preferred embodiment of the present invention is applicant's method and apparatus for handling low speed data communication between sites in a narrow band simulcast system. In accordance with a preferred embodiment of the present invention, the disclosed method and apparatus allows independent low speed data communication on each channel, provides more precise control of the low speed data synchronization and eliminates the need for any site paths dedicated to low speed data. Site controller computers can be optionally employed but are not required. A simulcast could operate in a "failsoft" configuration (see Rose et al. '396 patent) or have a site controller only at the main site or at each site. If a back-bone transmission system (i.e., the wide-bond stable communications link between the control point and the remote sites, for example a T-1 link ) failure occurs, normally, the remote sites would be shut down. (In the event of such a back-bone system failure, remote sites equipped with site controllers could be operated independently under the ,control of the system manager computer. However, care must be taken to prevent adjacent, overlapping sites from operating on the same frequency. In addition, a site must have at east two frequencies available to operate as a trunked site.)
Conventionally, in applicant's commercially successful wide band (e.g., 800 MHz) simulcast systems the following three distinct types of information signals are distributed from a control point site and controlled with the necessary precision required to provide simultaneous RF broadcasting at multiple displaced transmission sites: analog voice, low speed data and high speed data (which could be encrypted voice). High speed data is communicated to/from remote transmitting sites at 9.6 K baud via a multi-phase modem channel and adjusted for the appropriate RF transmission delay by digital delay circuitry and resynchronization circuitry at each site. Analog voice (clear voice) is communicated to/from remote sites on a separate delay corrected conventional FSK modem voice channel. Similarly, the low speed data, which conventionally is common to all channels, is converted to an audio band signal and handled as another "voice" path to each site.
It would be most efficient, desirable and cost effective if the above described existing "wide band" simulcast technology could be modified and utilized to provide narrow band simulcast capabilities for a PST system. However, narrow band (400 MHz) digital access communications presents certain difficulties in the above type of simulcast system. For example, the 9600 bps high data communications rate must be changed to 4800 bps. Although it is fairly straight-forward to modify existing 800 MHz simulcast system hardware to accommodate the new baud rates, efficient handling of the low speed data becomes a problem. The Channel Drop signal, which in the conventional 800 MHz system is at 4800 Hz and corresponds to the maximum high speed data rate of alternate ones and zeros, becomes a 2400 Hz signal which is audible (i.e., within the voice frequency transmission bandwidth). Low speed sub-audible data is therefore used for a channel drop signal and, thus, becomes unique to each channel. Unfortunately, this prevents using one common low speed data path to each site, and an additional path for each channel is neither practical nor feasible.
The present invention solves this problem by treating the low speed data as if it were a high speed data signal that happens to consist exclusively of long strings of binary "ones" and "zeros," then routing it to each site via the high speed data path dedicated for each channel. Since a channel is never used for transmitting high and low speed data simultaneously, it is possible to use the high speed data path between sites to carry low speed data whenever high speed data is not transmitted. Moreover, the high speed data path provides a greater degree of synchronization precision for the distribution of low speed data than obtainable via the FSK (voice channel) method conventionally used for distribution of the low speed data--primarily due to elimination of FSK modem quantization and the associated voice band filters. However, in the above described broad band simulcast system, the data resynchronization circuitry in the high speed data path at each site is designed to trigger upon detection of a long string of ones. Consequently, this resynchronization circuitry will constantly attempt to realign (delay) the low speed data transitions causing a corruption of the low speed data information content. Therefore, in order to prevent corruption of the low speed data in this manner, the resynchronization circuitry must be held at its previous latency during the distribution of low speed data (e.g., voice portions of the call).
In accordance with a preferred exemplary embodiment of the present invention, specific circuitry is provided to accomplish the switching (rerouting) of the low speed data onto the high speed data path and control of the resynchronization circuitry at both the control point site and the remote transmitting sites. In addition, an A/D (analog/digital) control signal, available at all sites in applicant's conventional simulcast system, is used in controlling timely path switching and "holding" of resynch circuitry.